A variety of techniques and systems are known for processing video signals to allow transmission of compressed video information between sites and for regenerating full bandwidth video signals at a receiving site. Intra-Field time-multiplexed video transmission (TMVT) is one technique, which offers good quality video performance at reasonably low cost. It will be appreciated that TMVT systems operate without producing temporal artifacts, since all processing takes place in the same video field. However, TMVT systems suffer from a loss of spatial resolution which results in somewhat blurry images reminiscent of consumer grade video tape. The video images encoded by TMVT systems is often characterized as being 37 soft."
Some of the "softness" exhibited by systems employing TMVT processing can be attributed to the distance between lines in an interlaced video field. The correlation between adjacent lines is an important factor in TMVT processing, since the high frequency portion of the video scene is 2:1 decimated on a line basis by the transmitter. The information for the missing lines is then generated by interpolation in the receiver. The fidelity achieved by interpolation is directly related to the correlation between the high frequency lines present in the signal arriving at the receiver.
Another factor affecting the perceived spatial resolution of the video image produce by TMVT systems is the use of vertical filters on both the transmitter and receiver sides of the system. Serial implementation of vertical filters seriously limits the vertical impulse response of the system. While operation without a transmit side vertical filter would leave the coder/decoder (codec) pair open to vertical aliasing due to the decimation process, this artifact would not seem to be serious, especially in a codec whose input is a comb-filter decoded NTSC video signal, since the comb filter is actually a vertical filter.
Yet another factor affecting the spatial resolution of the video image is the uneven error dispersal, which is quite evident in video images produced using TMVT processing. As shown, for example, in FIG. 1, all of the interpolation error is concentrated in one line of the two line pair which makes up the TMVT transmission format. When interlaced with the other field forming the NTSC video frame, the lines containing interpolation errors appear as pairs, producing an artifact that is generally characterized as "line crawl."
FIG. 2 illustrates a conventional intra-field TMVT transmitter, which is implemented using digital low-pass filtering of the input luminance signal. The TMVT transmitter 1 comprises a horizontal low pass filter 2 receiving a luminance signal and generating a half bandwidth signal, a subtractor 3 receiving the luminance signal and the half bandwidth signal and generating a high frequency signal corresponding to the high frequency component of the luminance signal, and a vertical low pass filter 4 receiving the high frequency signal and providing a filtered high frequency signal. Transmitter further comprises a demultiplexer 5 operatively connected to an adder 6 and a multiplexer 7, which is also connected to the output terminal of adder 6. It will be noted that adder 6 is also connected to filter 4 so that adder 6 receives both the filtered high frequency signal as well as the half bandwidth signal.
During operation, filter 2 generates a half bandwidth signal from the received luminance signal. The half bandwidth signal is subtracted from the luminance signal in subtractor 3, which yields the high frequency luminance component, i.e., the high frequency signal received by filter 4. This high frequency component is vertically filtered and 2:1 decimated, by techniques well known to those of ordinary skill in the art, and added to alternate lines of the low frequency luminance signal, i.e., the half bandwidth signal produced by filter 2 and supplied to adder 6 via demultiplexer 5. The resultant lines produced by adder 6 are of full bandwidth and are transmitted by a transmitter (not shown) as such. The other lines, which contain only half bandwidth signals, are horizontally 2:1 decimated, time compressed, preferably by multiplexer 7, and then multiplexed with the full bandwidth lines produced by adder 6 for transmission. The resultant signal produced by multiplexer 7 is an intra-field TMVT signal.
FIG. 3 illustrates a conventional intra-field TMVT receiver processor 10, which performs a complementary function, and which comprises a demultiplexer 11 connected to a horizontal low pass filter 12, a subtractor 14 and a multiplexer 17, all receiving the full bandwidth portion of the intra-field TMVT signal. Demultiplexer 11 is also connected to a time expansion circuit 13 which receives the half bandwidth portion of the intra-field TMVT signal. Subtractor 14 also receives a low frequency signal from filter 12 and generates a high frequency component signal provided to adder 16 via vertical low pass filter 15. As shown in FIG. 3, adder 16 is also connected on an input side to expansion circuit 13 and on an output side to multiplexer 17.
During operation, the received intra-field TMVT signal is demultiplexed in demultiplexer 11 into full and half bandwidth lines. The full bandwidth lines are filtered so that the high frequency part may be separated by subtractor 14 in a manner similar to that described above. The half bandwidth lines are horizontally 1:2 time expanded and filtered in expansion circuit 12. The high frequency component signal produced by subtractor 14 is then vertically interpolated from the adjacent lines and added to the expanded half bandwidth signal generated by expansion circuit 13. These interpolated lines, which are the lines produced by adder 16, are then multiplexed with the received full bandwidth lines from demultiplexer 11 by multiplexer 17, producing the resultant output luminance signal.